OSRAM Driving the Golden Dragon LED Application Note
Driving the Golden Dragon LED
e
Application Note
INTRODUCTION
The Golden Dragon LED is OSRAM Opto
Semiconductors’ high performance LED
requiring special considerations in thermal
management and electrical implementation.
This application note is intended to help the
design engineer with the special electrical
considerations of the Golden Dragon LED.
With a higher current there is higher power,
and therefore more heat to dissipate. The
Golden Dragon LED package is optimized
for removing this heat efficiently. With an
integrated heat slug (also known as a heat
spreader) the thermal performance is far
superior to standard LEDs.
Golden Dragon LEDs are delivered on tape
and reel. It has a flat top to allow pick-andplace machinery installation. All contacts
(including the heat slug) are soldered in
place using standard infrared reflow
processes (Surface-mount component
processing). Up to now the Golden Dragon
is the only high power LED in the market
which is capable to be processed according
to these cost effective standard assembly
techniques. ESD handling guidelines should
be followed when handling the Golden
Dragon LED.
Basic structure
Figure 1 shows the internal structure of the
Golden Dragon LED.
Leads
Dielectric
Figure 1 Structure of the Golden Dragon
LED
There are large leads for a strong mechanical attachment to the printed circuit board
(PCB), and to assist proper orientation of the
part during reflow soldering.
The semiconductor die is directly attached to
the heat slug. This heat slug is cast inside
the molding compound, which forms a
reflector cup around the die. The heat slug is
exposed on the bottom of the part for IR
reflow soldering to the PCB to provide a very
low thermal resistance from the die to the
PCB it is mounted to. The die is covered
with an optically transparent encapsulation
material to protect the die from the ambient
environment.
Bond Wir
Die Attach
Heat Slug
Die
Solder
Molding Compound
Solder Pads
Aluminum Plate
February 2, 2005 page 1 of 14
DESIGN CONSIDERATIONS
Thermal design
Because the Golden Dragon LED has a high
power rating, special consideration must be
made to optimize the thermal performance
of the entire system.
OSRAM Opto Semiconductors has released
an application note specifically addressing
thermal design for the Golden Dragon LED.
(The application note is titled “Thermal
Management of the Golden Dragon”). For
more details, please consult that application
note.
Optical design
The scope of this application note does not
include details of optical design. However it
is an important step in the lighting system
design and should not be ignored. Optical
design must target the highest efficiency to
reduce the LED light output requirements
and therefore the driver and heat-sink
requirements.
Electrical design
Semiconductor technology differences
There are two technologies used to produce
LEDs: InGaN and InGaAlP.
Different colors can be achieved with these
two technologies. InGaAlP is used to
produce colors from Green (570nm) to
Super Red (632nm). It has a forward voltage
around 1.8V to 2.3V, depending on the
color. InGaN is used to produce colors from
Blue (460nm) to True Green (528nm) and
phosphor based colors like White (typ.
3250K or typ. 5600K). It has a higher
forward voltage around 3.2V to 3.8V,
depending on the color. Be sure to check
the data sheet for the specific LED you are
using to get the correct information.
High current
The Golden Dragon LED is a high current
LED capable of operation at current levels in
the hundreds of milliamps. InGaAlP products
(Amber-Red and Yellow) can operate from
100mA up to 750mA. InGaN products (Blue,
Verde Green, True Green, and White) can
operate up to 500mA.
This high current develops a great deal of
power to dissipate in the LED. This power
can be up to two Watts in specific products.
OSRAM Opto Semiconductors will continually develop improvements to the Golden
Dragon LED. Please check the data sheets
for the latest performance data.
Steep If vs. Vf slope
The Forward Current vs. Forward Voltage
curve of the Golden Dragon LED is very
similar to any other LED. It has a steeper
slope of the I
area. This makes for rapid changes in
forward current with small changes in
forward voltage. The graph in Figure 2
shows this characteristic. Increasing the
current in the diode will not increase the
forward voltage by a significant amount.
0.5
0.4
0.3
0.2
Forward current (Amps)
0.1
0.0
0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2
Figure 2: Graph of Forward Voltage versus
Forward Current for a typical yellow Golden
Dragon LED.
vs. Vf curve in the high current
f
Forward Voltage (Volts)
February 2, 2005 page 2 of 14
Note that a 0.1 Volt change in forward
voltage is marked, and the indicated forward
current changes approximately 100mA. This
is approximately a 40% change in current
with a 5% change in forward voltage.
The intensity of the Golden Dragon LED is
closely linked to the forward current. With a
40% change in current, the intensity will
change approximately 40%. To properly
control the LED intensity, current control or
current limiting is mandatory.
Temperature coefficient of forward
voltage
All LEDs exhibit a change in forward voltage
as the junction temperature changes. This
temperature coefficient of forward voltage is
published in each data sheet of individual
LEDs. InGaAlP LEDs (Yellow and Amber
Red) have a coefficient of between -3.0mV/K
to -5.2mV/K, and the InGaN LEDs (Blue,
Verde Green, and White) have a coefficient
of between –3.6mV/K and -5.2mV/K. Check
the data sheet for the specific part you are
using to find this coefficient for your designs.
Intensity changes over temperature
variations
All LEDs also exhibit a change in intensity as
the junction temperature changes. This is a
result of changing efficiencies in the
semiconductor, and not a result of the
change in the forward voltage over temperature changes. This temperature change is
non-linear, but is represented in graph form
in all data sheets. Check the data sheet for
the particular LED you are using for this
graph.
Example of critical data sheet
information
The published Forward Voltage of the thin
film amber-red Golden Dragon LED (LA
W5SF) is provided as a minimum (2.05V), a
typical (2.4V) and a maximum (2.65V). This
is the range that the LEDs can be delivered
from production. This voltage is tested at a
specific current. It is best to use any LED as
close to the test current as possible. It is
important to verify operation over this
voltage range to be sure operation is in the
safe range.
The published thermal coefficient of forward
voltage for the thin film amber-red Golden
Dragon LED is -5.2mV/K. The published
maximum junction temperature of the Thin
film amber-red Golden Dragon LED is
125°C. It is important to verify operation over
the specified operating temperature range to
assure that the maximum junction temperature is not exceeded.
The published maximum current of the thin
film amber-red Golden Dragon is 750mA.
All conditions (input supply variations,
temperature variations, and production
variations) must be evaluated to assure the
maximum current is not exceeded.
HOW TO DRIVE A HIGH CURRENT LED
LIKE THE
LED circuit arrangements
Due to the high slope of the Forward Current
vs. Forward Voltage graph (Figure 2) it is
strongly recommended to only connect the
Golden Dragon LED in a series arrangement
with some current control for each series
string in the system. As described in the
application note titled “Comparison of LED
Circuits”, a matrix circuit has uncertainties in
the LED current that result from a mismatch
of the LED forward voltages. The Golden
Dragon LED will have this behavior but more
so.
Series resistor current limiting
Standard LEDs, like the Power TOPLED®,
typically employ a series resistor to limit the
forward current. With 350mA through a
series resistor, and a 12V supply, the
resistor power dissipation can easily exceed
3 Watts when used with a single Golden
Dragon LED.
GOLDEN DRAGON LED
February 2, 2005 page 3 of 14
Putting more LEDs in the string, and thus
A
−
=
reducing the resistor value, will reduce the
power dissipation in the series resistor. With
the small resistances resulting, the supply
voltage variations will cause larger current
variations in the LEDs. Figure 3 shows the
different effect on the current with supply
voltage variations. (A typical automotive
lighting application will see a variation from
9V to 16V)
400
350
)
300
250
200
LED current (m
150
100
50
9
10
11 12
Supply voltage (V)
1 Dragon
3 Dragons
4 Dragons
13
(37.5
(27.0
(18.0
14
)Ω
)Ω
)Ω
15 16
Figure 3: Comparison of LED current
variations with supply voltage variations
The smaller resistor creates a larger current
variation in the LEDs from the minimum to
the maximum supply voltage. This variation
in current will create a variation in light
output of the LED. There is a possibility that
the maximum forward current (as published
in the data sheet) will be exceeded when the
supply is at its maximum. To minimize this
variation, maximize the resistance by
reducing the number of LEDs in each string.
With high power LEDs, the resistor is kept at
a minimum to minimize power dissipation.
These are mutually exclusive requirements;
therefore a balance must be achieved with a
compromise. High power resistors can be
expensive, and assembly of a high power
resistor can add significant cost. (i.e. hand
soldering, selective soldering, clinching, antivibration mounting.) These factors must also
be considered when determining the
balance.
Example series resistor calculations
There are many factors that affect current in
the LED during operation:
Supply variation
First, the supply voltage has some variation.
(Typically 5% to 10%, automotive experiences a variation from 9V to 16V with
nominal being in the 12.5V to 13.5V range.)
As we discussed previously, the supply
variation can add a significant current
variation in the LEDs.
For example, let’s start with a low cost 5%
regulator supplying 12V (V
reduce the large voltage swings typical in an
automotive lighting application. If we put
three LEDs in a string, each with 2.4V (V
typical for a thin film amber-red golden
dragon LED), the series resistor will have a
4.8V (V
) drop at 0.350A (I
resistor
results in a 13.7Ohm resistor dissipating
1.68W (P
resistor
).
VnVV
*
diodesupplyresistor
resistor
R
R
resistor
V
resistor
=
I
diode
V
8.4
A
350.0
IVP
=
*
=−=
7.13
Ω==
dioderesistorresistor
==
At the limits of the regulator tolerance, the
supply voltage increases only 0.6V (V
12.6V maximum). The voltage dropped
across the resistor increases to 5.4V, and
the current increases by 0.044A. The LED
now passes 394mA.
). This would
supply
diode
VVVV
8.44.2*312
WattsAVP
68.1350.0*8.4
). This
supply
=
f
February 2, 2005 page 4 of 14
VnVV
*
resistor
diode
−=
VI
=
resistordiode
diodesupplyresistor
VVVV
4.54.2*36.12
=−=
Resistance/
AVI
394.07.13/4.5
=Ω=
Temperature Variation
The second factor affecting LED current is
the temperature coefficient of the forward
voltage of the LED. The data sheet for every
LED gives a coefficient for the forward
voltage with respect to the junction
temperature. At higher temperatures, the
forward voltage of the LED will decrease.
For the InGaAlP thin film amber-red LED,
the coefficient is –5.2mV/K.
KT
60
=∆
VKKV
3.060/0052.0
−=×−
VLEDsV
9.033.0
−=×−
With a temperature rise of 60K above room
temperature, the forward voltage of each
LED drops 0.3V. With three LEDs in a string,
(in an attempt to reduce power dissipation in
the series resistor) the forward voltage
across the string will drop 0.9V as a result of
the temperature change.
The effects of supply variation and temperature variation add. with a 5% tolerance on a
12V supply, and a 60K temperature
increase, there is a possible total variation of
1.5V across the series resistor. This
increases the current in the LEDs by a total
of 0.11A. The LED now is passing 0.46A.
Production variation
The third factor affecting LED current is
production variation of its forward voltage.
The data sheet of the Thin film amber-red
Golden Dragon LED gives a room
temperature forward voltage variation of
0.6V. With a design targeting the nominal
value, this can be seen as a ±0.3V
tolerance. This voltage change adds with the
first two effects creating a possible total
variation of 1.84V across the series resistor
in this application.
0.3V----Production
0.9V----Temperature
0.6V----Supply
0.3 + 0.9V + 0.6V = 1.8V
So, the voltage across the resistor can
increase by 1.8V. The current in the LED is
now 0.48A. This is not yet at the maximum
current published for the thin film amber-red
Golden Dragon LED, but heat dissipation at
this current level may cause the maximum
junction temperature to be exceeded. This is
still significantly above the design intent of
the LED. The design must account for this
much variation to prevent LED damage. The
power dissipated in the LED and resistor will
increase substantially, and must be taken
into consideration. The current could also
decrease when these tolerances move in the
opposite direction. If all the tolerances were
in the opposite direction, the LED current
would drop to 0.2A. This could create
problems from intensity variation and the
specification may not be satisfied.
Special consideration must be given to these
factors to be sure the LED’s maximum
current rating and the maximum junction
temperature are not exceeded at any time in
the application when using a series resistor.
This means the LED must be used at a
nominal level far below its capacity. Using
the Golden Dragon LED at a reduced
capacity with a series resistor will require
more LEDs. This can significantly increase
system costs. In most applications, the cost
saved by using only the needed Golden
Dragon LEDs and eliminating the special
assembly costs of a high power resistor, will
easily cover the cost of a current control
supply.
February 2, 2005 page 5 of 14
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